Clarify how signs' colours are derived.

README.mkd

@@ -270,7 +270,11 @@ endif
 
 #### Signs' colours and symbols
 
-To customise the colours, set up the following highlight groups in your colorscheme or `~/.vimrc`:
+By default vim-gitgutter uses your colourscheme's `Diff*` highlight groups' foreground colours for the signs' foreground colours.  For example, your `DiffAdd` foreground colour will be used for the `+` sign's foreground colour.
+
+The signs' background colours will all be set to the sign column's background colour.
+
+If you don't like the default colours, you can either fix your colourscheme's `Diff*` highlights or configure your own `GitGutter*` highlight groups.  These groups are:
 
 ```viml
 GitGutterAdd          " an added line             (default: links to DiffAdd)
@@ -281,6 +285,16 @@ GitGutterChangeDelete " a changed line followed by at least one removed line (de
 
 You can either set these with `highlight GitGutterAdd {key}={arg}...` or link them to existing highlight groups with, say, `highlight link GitGutterAdd MyDiffAdd`.
 
+To get vim-gitgutter's original colours (based on git-diff's colours in my terminal):
+
+```viml
+highlight GitGutterAdd    guifg=#009900 guibg=<X> ctermfg=2 ctermb=<Y>
+highlight GitGutterChange guifg=#bbbb00 guibg=<X> ctermfg=3 ctermb=<Y>
+highlight GitGutterDelete guifg=#ff2222 guibg=<X> ctermfg=1 ctermb=<Y>
+```
+
+– where you would replace `<X>` and `<Y>` with the background colour of your `SignColumn` in the gui and the terminal respectively.  For example, with the solarized colorscheme and a dark background, `guibg=#073642` and `ctermbg=0`.
+
 To customise the symbols, add the following to your `~/.vimrc`:
 
 ```viml